Abstract: Both temporal and spatial gradients of ionosphere are major factors that influence the phase smoothing of the carrier for GBAS.In order to reduce the effect of ionosphere on the pseudo-range smoothing output and improve the availability of the differential correction on smoothed pseudo-range,a cascaded dual-frequency smoothing filtering algorithm based on the analysis of additional ionosphere error induced by single frequency phase smoothing is proposed.The filter algorithm estimates effectively the ionosphere delay in the navigation signal through the first-order low-pass filter,and takes code minus carrier (CMC) generated through linearly combining the first-order filter output with dual-band signal pseudo-range value and the carrier phase value as a second-order low-pass filter input,with correcting the output of the smooth pseudo-range value of the clock correction,eliminates the ionosphere delay error and attenuate the received signal noise.The results show that the improved algorithm can effectively reduce the influence of the ionosphere on the pseudo-range differential correction and the residual error of the smoothed pseudo range,improve the availability of the smoothed pseudo-range differential correction,and the positioning accuracy during the precision approach landing.

Key words: ionosphere, smoothing filter, availability, differential correction, cascade dual frequency